The Shulhan Aruch writes that when three or more people eat together and recite a Zimun before Birkat Ha’mazon, they should use a cup of wine for Birkat Ha’mazon. This means that the one who leads the Zimun holds a cup of wine during Birkat Ha’mazon, and then after Birkat Ha’mazon, he recites the Beracha of "Gefen" and drinks the wine.
Unfortunately, many people do not observe this practice. It is important to understand that just as we use a cup of wine at a Huppa, Berit Mila and Pidyon Ha’ben, as befitting these special Misvot, a cup of wine should be used also for Birkat Ha’mazon, as it, too, is a very precious Misva.
The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) writes that if one has only one cup of wine at the end of Shabbat, and he must choose between using it for Birkat Ha’mazon and for Habdala, he should use it for Habdala. And in a case where one does not have a cup of wine for Birkat Ha’mazon, he can use "Hamar Medina" – a different intoxicating beverage which is regarded as an important beverage in one’s society, an example of which in our society would be beer. Of course, before drinking the beer one recites "She’ha’kol," and not "Gefen."
One adds three drops of water into the cup that is used for Birkat Ha’mazon, just as we add three drops to the Kiddush cup before Kiddush – a practice called "Meziga." In the case of Birkat Ha’mazon, however, the drops are added not before the beginning of Birkat Ha’mazon, but rather after reciting the words in the second blessing, "Ve’achalta Ve’sabata U’berachta." At the time one recites the next word – "Et" – he should add the three drops (as printed in some editions of the Siddur).
The Ben Ish Hai writes that the water used for "Meziga" should be fresh, and not water from which one has drunk. A cup of water from which one has drunk is considered "Pagum" ("defective"), and should not be used for "Meziga." People sometimes make the mistake of taking leftover water from their glass at the table for the three drops of "Meziga." One should instead prepare a new cup with water before Birkat Ha’mazon to be used for the "Meziga" during Birkat Ha’mazon.
The Ben Ish Hai also writes that when performing "Meziga," one should have in mind the Gematria (numerical value) of "Mayim" (water), which is 90. This alludes to the nine instances of the letter "Yod" (which in Gematria equals 10) in four certain Names of Hashem. For example, the special 72-letter Name of G-d is expressed by writing out the names of the four letters that comprise the Name of "Havaya" – "Yod," "Heh," "Yod," and "Heh." The Gematria of the word "Yod" (spelled "Yod," "Vav," "Dalet") equals 20; the Gematria of "Heh" (spelled "Heh," "Yod") equals 15; the Gemara of "Vav" (spelled "Vav," "Yod," Vav") equals 22; and the Gemara of the second "Heh," of course, is 15 – for a total of 72. This series of names of letters contains a total of four instances of "Yod." There are three other Names of G-d which, when configured in this fashion, have a total of five instances of "Yod," and thus in all, these four Names contain nine "Yods." This is the allusion of the water ("Mayim" = 90) which we add to the cup of wine.
It must be emphasized that the Ben Ish Hai’s work in which he wrote this allusion was intended for laymen, and not for Kabbalists. This means that he felt that all people should have this in mind when performing "Meziga," and this is not something reserved for those trained in the esoteric secrets of Kabbalah.
Moreover, this shows us the great significance of the cup of wine used for Birkat Ha’mazon, and of "Meziga," which has the effect of bringing Names of Hashem into the wine. Indeed, it is customary for the wife to drink some of the wine from this cup as a Segula for fertility. The cup of wine used for Birkat Ha’mazon is also considered beneficial for health.
In conclusion, it is worth noting the comments of the Aruch Ha’shulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908), who lamented the fact that most Ashkenazim do not observe this practice of reciting Birkat Ha’mazon over a cup of wine. He attributed this phenomenon to the abject poverty which Ashkenazic Jewry endured for many generations, during which time wine was not easily accessible, and even obtaining wine just for Kiddush on Shabbat was often a challenge. Under these conditions, it was not possible to use a cup of wine every time Birkat Ha’mazon was recited, and so Asheknazic communities gradually dropped this practice. However, the Aruch Ha’shulhan writes, in modern times, when wine is readily available, there is no reason not to observe this precious custom of reciting Birkat Ha’mazon over a cup of wine whenever a Zimun is recited.
Summary: Whenever a Zimun is recited before Birkat Ha’mazon, it is proper for the one leading the Zimun to recite Birkat Ha’mazon over a cup of wine. A fresh cup of water should be prepared before Birkat Ha’mazon, from which three drops are taken and added to the cup of wine after reciting the words "Ve’achalta Ve’sabata U’berachta" in the second blessing of Birkat Ha’mazon. When adding the drops of water, it is proper to think that the word "Mayim" has the numerical value of 90, alluding to the six instances of the letter "Yod" in four certain Names of Hashem.
